Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish or similar-sized pan.
Prepare the fruit filling: In a large bowl, combine mixed fruit, sugar, lemon juice, cornstarch, vanilla extract, and cinnamon. Toss until evenly mixed. Pour the fruit mixture into the prepared baking dish.
Prepare the biscuit topping: In a separate bowl, whisk together flour, sugar, baking powder, and salt. Add cold cubed butter and use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture resembles coarse crumbs.
Stir in buttermilk (or regular milk) until just combined. The batter will be thick.
Assemble the cobbler: Drop spoonfuls of the biscuit topping over the fruit filling, leaving some gaps between the spoonfuls.
Bake for 40–45 minutes, or until the topping is golden brown and the fruit filling is bubbling.
Allow the cobbler to cool slightly before serving.